The present invention was developed to provide improved operation and longer life for plasma arc remelting systems used to make metal ingots and components of such systems. Together with the improved structure the invention contemplates improvements in the methods for the operation of such systems.
Installations in the prior art used for the production of ingots in plasma arc remelting teach use of a cooled mold with a vertically movable bottom part for lowering the ingot being made. The mold is positioned within the lower portion of a hermetically sealed chamber and the installations utilize one or several plasma torches connected to a source of electrical energy. A suitable power operated mechanism connected to the bottom part provides for moving that part and extracting the formed ingot. Reference can be made to U.S. Pat. Nos. 3,147,329 and 3,496,280 for explanations of plasmatron operation and plasma arc remelting.
One known installation of this type is disclosed in British Patent 1,237,155 based, in part, on prior development work of several of the applicants hereof. A serious problem encountered was that the plasma arcs frequently burned through the water cooled torches and/or the mold, thereby releasing the coolant fluid into the evacuated space of the chamber and causing serious explosions due to the presence of high temperature molten metal therein. In that installation plasma torches having a fixed position with respect to the mold were provided for melting a metal blank which was lowered into the remelting chamber. Difficulties with the thermal balance in this installation were encountered and overcome. Another problem was that the plasma arcs did not occupy the same paths between the plasma torches and the mold in successive runs. As a result the metal blank was not uniformly melted in this apparatus.
Many of these disadvantages were overcome by development of the improved system disclosed in part in a U.S.S.R. publication entitled Stahl, No. 6, 1971 which teaches top feeding and revolving of a blank in a plasmatron furnace as well as angular adjustment of at least one of several torches arranged to direct the plasma arc flame downwardly toward the lower end of the blank and against the upper end of the water-cooled mold. Those improvements enabled operation wherein at least one of the plasma arc torches is adjustably mounted via a ball and socket joint in the chamber so that the position of its plasma arc flame can be adjusted with respect to the mold and wherein the metal blank being melted can be rotated as well as lowered axially into the remelting zone within the chamber.
Radial arrangement of several plasmatrons around a crystallizer allows the placement of heat sources evenly around the molten pool, or bath, of metal which exists at the top of the ingot being formed. Precise regulation of the heating of all sections of the bath is obtained by changing the circumferential distances between the plasmatrons. It is known, that at low remelting rates, 70 to 80% of the heat released by the solidificating ingot is removed to the water-cooled copper crystallizer through its contact strip with the bath. In heating the bath by plasma flames placed along the periphery of the bath it is easy to obtain its flat shape. Also, at a certain inclination of the plasmatrons to the bath, one can make the liquid metal revolve, at a desired rate, around the vertical axis by using the energy of plasma jets.
The experience in operation of plasma-arc furnaces with a radial arrangement of plasmatrons around the crystallizer has shown, that through control of the heating of the bath by changing the peripheral distances between the plasmatrons, one can obtain in the same furnace (by changing only the crystallizer and priming) round, square, rectangular and other shaped ingots from the same blank, for instance, of round cross section.
Another significant advantage of the multi-plasmatron furnace with axial feed of the blank is an essential (almost 70%) radiation screening of the plasma jets and bath by the blank. Blanks larger than 150 mm in diameter are melted close to free surface of the bath and their melted face takes a falt or a concave form, thus causing an increase in the efficiency of the remelting process. In this case the demands put on the quality of the blanks are less rigid than on blanks used in furnaces with sidewise feed of blanks, where usually the blanks must be much thinner than the ingot and therefore their manufacture consumes more labor. Blanks for multi-plasmatron furnaces can be of round or square cross section, or they can be composed of end and side scrap of sheet. In the case of remelting a loose material in a furnace with a radial arrangement of plasmatrons, the material is fed to the middle of the bath, securing a good and complete melting of the fed material.
A non-uniform temperature field is generated during melting of a metal in a water-cooled copper crucible by intensive heat-energy flow from plasmatrons. Temperature gradients can reach 200.degree./cm. The non-uniform temperature field generates free-convexion macroflows, causing a stirring of the metallic bath with an intensity directly proportional to the number of plasmatrons installed in the furnace. This stirring promotes a chemical homogenization of the molten metal and accelerates the reactions which take place in the diffusion zone. Therefore, in the multi-plasmatron furnaces ingots of higher quality can be obtained than in single-plasmatron furnaces, not only because of the thermal conditions of the process, but also in connection with a more favorable diffusion kinetics of metal-refining reactions.
Experience gained from those previously known plasma arc remelting systems brought to light various difficulties in obtaining appropriate control over torch adjustment, operational functioning of the feed and revolving structures and emphasized the very short torch life, one of the major problems of plasma arc torches used in furnace systems for remelting metals.
The location of plasmatrons around the crystallizer of the furnace does ensure better operational conditions than in the case of axial arrangement, where the whole, or almost the whole, capacity of the furnace is concentrated in a single plasmatron. Nevertheless, to be economically feasible and acceptable, the plasmatrons used in metallurgy, whether in single or multi-torch furnaces, unlike plasma generators for welding, cutting, surfacing, etc., must possess a considerable resource in working capacity and be reliable in operation; in this respect the ablation of the tungsten cathode and the failure of the nozzle must be eliminated or reduced to a minimum. The stronger is the current of the power source, the more difficult it is to secure high working capacity of the plasmatron. Prior art plasma arc torches have been water cooled, the nozzles have been water cooled and even the center electrode has been water cooled but still the tunsten cathodes and the nozzle structures fail in a short time, often before an ingot is completed.